Wireless charging and patient monitoring are becoming increasingly important in medical applications. By incorporating wireless charging technology, medical implants benefit from greater site flexibility, a smaller total footprint and reduced battery size. However, many hurdles still exist for implant devices utilizing inductively based telemetry, including antenna size constraints, the limited number of antennas that may be included in an implant, and the expected attenuation caused by tissue that further weakens the inductive coupling link. All of these factors must be taken into account to achieve reliable patient monitoring, consistent actuation of therapy (e.g., drug delivery, electrical stimulation, etc.) and inductive power transfer for charging the implant battery.
Telemetry applications include pacemakers, medicine delivery pumps, stimulation devices, and artificial hearts. Implantable drug delivery systems, for example, which may have a refillable drug reservoir, cannula and check valve, etc., allow for controlled delivery of pharmaceutical solutions to a specified target. This approach can minimize the surgical incision needed for implantation and avoids future or repeated invasive surgery or procedures. Refillable ocular drug pumps, for example, usually hold less than 100 μL, are much smaller and more difficult to access post-implantation than other implantable pumps, such as those used for intrathecal injections or insulin therapy.
Thus, an implantable drug-delivery pump may incorporate telemetry to facilitate communication with an external monitoring device and wireless charging of the battery powering the implanted device via inductive coupling. The operating parameters of the implantable pump may be non-invasively adjusted and diagnostic data may be read out from the pump to the external monitoring device through wireless signals. During a scheduled visit, a physician may place the monitoring device near the implantable pump and send wireless signals to the implantable pump. The implant, in turn, adjusts the parameters in the pump and transmits a response command to the monitoring device. Typically, a medical telemetry device comprises a coil antenna that transmits and receives signals using electromagnetic waves. However, other antenna configurations known in the field may be utilized as well. A number of parameters characterizing the efficiency of the coil antenna, e.g., the resonant frequency, gain, quality factor (Q factor), and the thermal effect (Joule effect or heat) are considered when selecting or designing the coil antenna.
The wireless power receiver system may comprise additional electronic components such as a battery, a magnetic core, and circuitry including data storage and a transceiver for data. Some or all of the circuitry is usually hermetically sealed within a device case, but the telemetry coil may be placed externally to mitigate any interference caused by certain case materials. Achieving sufficient power transfer across tissue to an implanted device can pose a major challenge, particularly for small devices.
FIGS. 1 and 2 highlight the difference between a small implanted system with telemetry and a traditional system such as a radiofrequency ID (RFID) or similar broadband system, in which a tag or other inductively responsive device is powered and interrogated wirelessly by a reader. As shown in FIG. 1, in a broadband system, the bandwidth of the receiver (i.e., the tag) is sufficiently large for the response to be almost flat across the transmitter bandwidth, even accounting for tolerances; that is, while the degree of inductive coupling is not especially high, it is consistently well above zero across a large frequency band. Consequently, even if the peak transmission frequency varies, it will still transfer power and/or data to the receiver. Comparatively, in a narrowband inductive link such as those employed by small implantable devices, the receiver bandwidth is narrow. Hence, as shown in FIG. 2, it may fail to coincide (or coincide sufficiently) with the transmitter bandwidth, accounting for tolerances, to facilitate adequate power and data transfer. At the same time, when the transmission and frequency bands do coincide, the system achieves much higher normalized gain.
The resonance frequency (in Hertz) of an inductive link is controlled by a capacitor of capacitance C in parallel with the receiving coil of inductance L, and its value is given by the following equation:
      f    0    =      1          2      ⁢                          ⁢      π      ⁢              LC            
A narrowband system is very sensitive to proper tuning (i.e., the disparity in f0 between receiver and transmitter) and susceptible to any shift of resonance frequency or transmitter frequency drift. The result of a frequency mismatch in broadband and narrowband systems is illustrated in FIG. 3. In a broadband system, the degradation in coupling efficiency remains low across a relatively wide range of frequencies, whereas in a narrowband system, the coupling efficiency drops drastically with even a modest mismatch in resonance frequencies. The faster the drop-off, the more limited will be the practicability of implementing a narrowband system given realistic manufacturing tolerances and inevitable drifts during operation, as illustrated in FIG. 4, which shows the relative effects of detuning or oscillator drift in broadband and narrowband links. As a practical matter, the lower limit of usability is reached when the efficiency falls below 70%, and for the system represented in FIG. 4, this is a frequency mismatch of merely ±1 kHz.
Accordingly, traditional RFID design principles do not readily apply to very small devices utilizing narrowband links. An RFID transmitter is adjusted so that its frequency matches the resonance frequency to obtain maximum power transfer. Oscillator frequency drift or detuning effect is typically dealt with by minimizing its amplitude by design (choice of components, tight tolerance of components) and building the necessary margin into the link budget to cope with the resulting degradation. For a well-designed system, less than 50% degradation can be achieved. In order to maximize the power received for a given receiver coil or minimize the size of the receiver coil for a given power target, the bandwidth of both the transmitter and receiver are narrowed to what is required to still maintain data communication a few kHz in typical implementations. This ensures the maximum possible combined Q factor and therefore the maximum power transfer. Because the bandwidth of both the transmitter and receiver may only be a few kHz, their resonance frequencies may need to be accurately matched to avoid significant coupling degradation.